1. Field of the Invention
This invention relates to polyorganosiloxane compositions. More particularly, this invention relates to liquid polyorganosiloxane compositions exhibiting improved flow properties and workability and which cure to form a gel.
2. Description of the Prior Art
Liquid polyorganosiloxane compositions that cure to form gel materials when cured by the reaction of silicon-bonded hydrogen atoms with vinyl radicals are known in the art. Compositions of this type are disclosed, for example, in U.S. Pat. No. 3,284,406, which issued to M. Nelson on Nov. 8, 1966, and in U.S. Pat. No. 4,100,627, which issued to A. E. Brill on July 18, 1978. The flow properties of prior art gel-forming compositions could make them difficult to use in a coating operation. Those compositions exhibiting low viscosities in the uncured state are easily applied as coating materials, however relatively thick films cannot be readily achieved in a single application due to the highly flowable nature of the compositions. At the other extreme, high viscosity compositions would be difficult to apply as thin, continuous, uniform films. For certain end uses requiring coating of selected small areas on a substrate, coating of electrical wires, or the application of relatively thick coatings, it is desirable that the gel-forming polyorganosiloxane composition exhibit a low viscosity during application and a low degree of flowability after application.
This invention provides an ideal gel-forming polyorganosiloxane composition for coating operations in which the above-mentioned shortcomings of conventional gel-forming polyorganosiloxane compositions are improved. In other words, according to this invention, the uncured polyorganosiloxane is thixotropic, in that it is characterized by a low apparent viscosity under shear stress, also referred to as shear deformation stress, during application, and by a high post-application apparent viscosity in the absence of shear stress. Following curing, the composition is a soft gelled elastic material.